1. Field of the Invention
The present invention relates to medical imaging systems; more particularly, the present invention relates to silicon photomultipliers (SiPMs) in medical imaging systems.
2. Description of Related Art
SiPMs are semiconductor photon sensitive devices made up of an array of very small Geiger-mode avalanche photodiode (APD) cells on a silicon substrate. An example 10×10 microcell array is shown in FIG. 1. Each cell is connected to one another to form one larger device with one signal output. The entire device size can be as small as 1×1 mm or much larger.
APD cells vary in dimension from 20 to 100 microns depending on the mask used, and can have a density of up to 1000/sq. mm. Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high-speed telecommunications using optical fibers, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe operates in the infrared, to a maximum wavelength of about 14 μm, requires cooling to reduce dark currents, and can achieve a very low level of excess noise.
Silicon avalanche diodes can function with breakdown voltages of 100 to 2000V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 100-200 V in silicon). Greater voltage can be applied to silicon APDs, which are more sensitive compared to other semiconductor photodiodes, than to traditional APDs before achieving breakdown allowing for a larger operating gain, preferably over 1000, because silicon APDs provide for alternative doping. Reverse voltage is proportional to gain, and APD gain also varies dependently on both reverse bias and temperature, which is why reverse voltage should be controlled in order to preserve stable gain. Silicon PMTs can achieve a gain of 105 to 106, by operating with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level.
Geiger-mode APDs produce a large, fast pulse when struck by a photon of the same amplitude no matter the energy of the photon. When many of these cells are placed together in an array, they can be combined into one large array which will produce an output pulse proportional to the input photon pulse. This device is referred to as a SiPMT. However, as the size of the array increase, so does the capacitance and noise of the device. The supply voltage needed for a SiPM device varies from 30V to 100V depending on the junction type, and is less than the supply voltage needed for a PMT by a factor of from 30 to over 60. The capacitance and noise are proportional to the area of the SiPM device. The risetime of the device is also proportional to its capacitance, and the risetime and noise are the major factors in determining the time resolution in PET. The timing resolution degrades if the risetime becomes longer and the signal becomes noisier. Therefore, the optimal SiPM device would be a very small, fast, low noise device.
However, the smaller the device is, the fewer photons that can be collected to be used for the 511 keV energy discrimination. Thus, typically the size of the device needs to be compromised, resulting in a device that is as large as needed for adequate light collection and energy resolution.